Recently, along with the increase of the capacity of an optical communication system, a digital coherent communication scheme using multivalued phase amplitude modulation has become increasingly popular. This communication scheme is a scheme to transfer a digital signal using optical phase information, and thus, a laser light source with less phase noise and a narrower spectral linewidth is required for a light source from which carrier light is supplied.
There are various kinds of laser light sources, but since a semiconductor laser is compact and of low cost, it has become widespread as a light source for optical communications. In particular, in a mid- and long-distance optical communication system, a Distributed FeedBack type (DFB) laser and the like which operate in a single mode is widely used. Further, in the mid- and long-distance optical communication system, Wavelength Division Multiplexing (WDM) technology is generally used for increasing the capacity of transfer per optical fiber. Therefore, a wavelength tunable characteristic capable of outputting an arbitrary wavelength channel is also required for the light source for making digital coherent communications.
Since the semiconductor laser has a resonator of a small size compared to other types such as a solid-state laser and a gas laser, its phase noise is relatively large. For example, a linewidth of the semiconductor laser having the general resonator size of several hundreds of μm is in MHz order. Here, the linewidth refers to a spectral linewidth, and is represented in Full Width at Half Maximum (FWHM) of a spectrum. In a digital coherent communication system of 100 Gbit/s which has presently been widespread, a Quadrature Phase Shift Keying (QPSK) modulation scheme is used, and a spectral linewidth of several hundreds of kHz is required for the laser light source. For such a purpose, a wavelength tunable DFB laser array and an external cavity type laser having a resonator length extended to about 1 mm so as to narrow the spectral linewidth, for example, are used. For realizing large volume of communications by using a modulation scheme with a greater multivalued degree, it is expected that the light source of a further narrower spectral linewidth will be implemented in future. In addition, also for application to optical sensing other than the communications, the light source of a narrower spectral linewidth is required to observe a narrow absorption line spectrum with high sensitivity.